This invention relates to a vibration damping material for application to the surfaces of vibratory structures, the vibration damping material being of the type containing a flake filler dispersed in a polymer matrix.
With the progress of high-speed traffic and mass transport, evil influences of vibrations and noises in various aspects have become a serious social problem. In some urban communities where many people reside in apartments, even domestic noises and vibrations in the neighboring homes are threatening the health of the residents. Therefore, now it is an important and urgent social demand to reduce the vibrations of machines, structures and so on and noises attributed thereto to thereby recover and maintain a tranquil and pleasant social environment.
Extensive researches on damping of mechanical vibrations have been made as well as researches on other methods for reduction or suppression of vibrations, and various kinds of vibration damping materials have been developed until now. In general these vibration damping materials are viscoelastic materials in which an organic polymeric material is used as the principal component, and in some cases organic polymer base composite materials are used. For damping of the vibration of a plate, for simplicity, a vibration damping material is brought into direct contact with a surface of the plate to form a coating layer of a suitable thickness. As is well known, the two basic types of vibration damping using a viscoelastic coating layer are the extensional type and the constrained shear type. In the case of the extensional type the outer surface of the coating layer of the viscoelastic damping material is left uncovered, but in the case of the constrained shear type a stiff constraining layer or plate is provided on the outer surface of the viscoelastic layer.
A vibration damping material for use in the extensional type damping is required to be large in the loss modulus, whereas a vibration damping material for use in the constrained type damping is required to be large in the dynamic loss factor. In either case it is also required that the loss modulus or the dynamic loss factor be sufficiently large over a wide temperature range and over a wide range of frequencies. As to the polymeric materials conventionally used for vibration damping, it is usual to utilize the glass transition region of each polymeric material since the loss modulus or the dynamic loss factor becomes largest in that region. To obtain a polymeric material having a relatively broad glass transition region, sometimes blending of two or more kinds of polymers which are miscible with each other or cross-linking copolymerization (or synthesis of interpenetrating polymer network) of nonmiscible polymeric materials is employed. In the cases of vibration damping materials for use in the extensional type damping, often an inorganic powdery, granular, flaky or fibrous filler is added to an organic polymer used as the matrix material to obtain a composite material that exhibits a relatively large loss modulus.
However, the conventional vibration damping materials are still unsatisfactory or disadvantageous in certain points. As to the conventional damping materials that utilize only an organic polymer, whether a single kind of polymer or a plurality of kinds of polymeric materials, the loss modulus or the dynamic loss factor of each damping material greatly and sharply decreases in a temperature range above the glass transition temperature or in a low frequency region. As to the conventional damping materials containing an inorganic filler with the intention of using in the extensional type vibration damping, the actual effect of the filler is not as high as expected. Besides, an increase in the loss modulus as the effect of the addition of an inorganic filler to an organic polymeric material is accompanied by a decrease in the dynamic loss factor, so that the damping materials of the composite type are of little use for vibration damping of the constrained type. From an industrial point of view, flexibility of the supply of the composite damping materials is marred by such restriction on the manner of use.